(1) Field Of The Invention
The invention relates to a process for pretreating the surface of a single-layer or multilayer molded material by means of an electrical corona discharge which takes place between high voltage electrodes and a grounded counter-electrode, between which the single-layer or multilayer molded material is located, and an apparatus for pretreating the surface of the molded material. The present invention is particularly suitable for electrically conducting molded materials.
(2) Prior Art
In many cases, the normally smooth surface of molded plastic materials, in particular of films, presents difficulties in that the film webs have extremely good slip and readily tend to telescope when being wound on a roll. Further difficulties arise during treatment of these materials for increasing the adhesion of printing inks, coatings, adhesives, metals applied by vapor deposition, and the like due to the smooth surfaces of films or molded materials. The prior art for overcoming these difficulties includes physicochemical surface modification of plastics, in particular of films. A basic process which only produces changes on the surface of the plastic consists of pretreating the surface of the plastic by means of an electrical corona discharge.
According to German Offenlegungsschrift No. 3,247,795, corona pretreatment of a film web of plastic is carried out by a procedure in which the upper surface and/or the lower surface of the film web are exposed to a corona which results in different pretreatment intensities. For this purpose, the film web to be treated is fed over an electrically grounded roller, and electrical charging is effected by exposure of that surface of the film web which faces away from the roller surface to an electrical corona discharge. The electrical corona discharge is generated by applying a high-frequency alternating current of high voltage to an electrode spaced a small distance away from the roller. The pretreatment is carried out in general in air under atmospheric pressure.
The constantly increasing market requirements of products having improved surface properties have also led to the development of processes using chemically reactive substances which, for example, cleave certain chemical bonds in the surface and thus alter the surface properties of plastic. U.S. Pat. No. 3,142,630 describes a process for increasing the adhesion, in which a film web is passed through a non-ionizing liquid and is exposed to a corona discharge in the liquid. This liquid may be, for example, a transformer cooling oil, vegetable oil or another pure oil, which is free from impurities and which is substantially electrically nonconducting.
British Patent No. 938,325 describes a process for the pretreatment of thermoplastic films, in which an electrical corona discharge takes place at the surface in a nitrogen atmosphere. The nitrogen is passed via distribution lines, through hollow electrode lines, into the corona discharge zone.
In the arrangement described in U.S. Pat. No. 3,274,089, organic compounds from the group consisting of polymerizable organic compounds, non-polymerizable organic compounds having substitutable hydrogen atoms and perhalogenated hydrogen are passed into the corona discharge zone through distribution lines in order to modify the surface of film webs or articles made of polymers.
The common feature of these known processes is that reactive gases are passed into the corona discharge region between the electrodes, or the corona discharge is allowed to take place in a nonconducting liquid.
Japanese Patent No. 17 747/73 discloses an apparatus in which a film surface is exposed to a corona discharge. The electrode connected to the generator consists of porous sintered metals and of a plurality of metal nets. The high voltage discharge electrode is shaped in such a way that liquid fed into it accumulates and is stored. As a result of the voltage applied to the discharge electrode, the stored liquid is converted into the gas phase and emerges from the porous sintered metals in the form of gas particles, which move toward the film surface under the influence of the electric field lines of the corona discharge.
In the apparatuses and processes which include a liquid as the discharge electrode in the corona discharge process, it is necessary to rely on special apparatuses which permit storage or accumulation of the liquid and furthermore must consist of a material which allows the liquid converted into the gas phase to pass there through into the corona discharge zone. If the film web to be pretreated is passed through a liquid in which the corona discharge takes place, the transport velocity of the film web through the liquid is obviously limited. If a corona discharge is effected in a reactive atmosphere on the surface of the plastic, various layers can be subsequently applied by further process measures to treat the surface of the plastic. Coating simultaneously with the pretreatment is not possible in such a case. The same applies to pretreatment of surfaces of plastics where the corona discharge is carried out on the surface in a liquid.